The patents of Jongsma (U.S. Pat. No. 5,406,342), Warnicke, et al.(U.S. Pat. No. 4,995,716) and Baron (U.S. Pat. No. 4,761,071), present inventions for systems and methods of projecting image patterns onto the cornea of the eye and then by means of a video camera, measure the shifting of elements of the image pattern and from the measurements compute the three dimensional surface topography.
Jongsma utilizes a frequency domain concept called "Fourier Profilometry" which has been implemented by Euclid Systems and, like the direct geometrical approach of the Warnicke invention implemented by Par Technology, employs the technique of rastostereography wherein a pattern is flash projected on the full surface of the anterior cornea to which a fluorescein dye has been applied. Because the light from the flash lamp is filtered to pass light only in the blue/violet wavelength region, the fluorescein dye causes the projected light to fluoresce to the wavelength of yellow light. Then a narrow bandpass filter allows only the diffuse reflected yellow light from the projected raster (or grid) image to reach the video camera. This prevents the blue/violet light reflected from the optically smooth surface, known as the specular surface, of the cornea from interfering with the imaging of the desired diffuse projected grid.
The two systems are in prominent usage and achieve topography measurement accuracies on the order of a few microns. Although both have been mentioned in the application of measuring corneal topography during photorefractive keratectomy, the problem of specular reflection has not been addressed in detail. For these systems to be used on surfaces such as would be encountered during the process of corneal photoablation where the epithelium has been removed to expose the stromal layer, no fluorescein dye is used and the light filters are removed. The problem that arises in this application is that unless the corneal surface totally diffuses the light (similar to a movie projection screen or a mat finish paper) remanent regular or specular reflected light from the semi-diffuse corneal surface can occlude portions of the diffuse image pattern when it is captured by the video camera.
Contemporary applications of these two systems, also to Placido disk type systems, are almost exclusively limited to single measurements of corneal topography for the purpose of ophthalmologic evaluation and/or contact lens fitting. Therefore, the several seconds they require to perform a topographic measurement is adequately rapid. However, for application to corneal surgery for refractive correction or therapeutic procedures, a real time, nearly continuous, topographic display is either necessary or desirable. The objective of such a system would be to provide to the ophthalmic surgeon a differential surface of the cornea showing micron level departures from the reference surface at frame rate indistinguishable from real time. Such performance was contemplated in my U.S. Pat. No. 5,350,374, but was limited in that it did not address the need to correct the specular reflected light problem and also was susceptible to errors caused by saccadic eye movement.
To create a system and method enabling real time topography measurement of semi-diffuse surfaces, the following objectives must be achieved: Preventing specular reflected light from interfering with the diffuse reflected light of the projected grid image; maximizing the light gathering power, the depth of field focussing capability, the resolving power and the image capturing speed of the video camera imaging system; minimizing the effect on measurement accuracy due to variations in the target object distance from the video camera; overcoming computational speed limitations of the prior art so that many full surface topographic measurements may be made per second.